Surface-plasmon-generated light source and its use

ABSTRACT

A structure for producing a localized light source in a medium is provided. The structure includes a source generating incident light, a surface-plasmon-supporting layer, and means for transmitting and localizing plasmons between the surface-plasmon-supporting layer and the medium. The transmitter-localizer means includes between the surface-plasmon-supporting layer and the medium a discontinuity for providing a localized electromagnetic field deviation and a plasmon-transmitting interface with predetermined electromagnetic properties at the medium. The incident light excites a surface plasmon in the surface-plasmon-supporting layer, which plasmon in turn produces the localized light source at the plasmon-transmitting interface by localizing the energy of the surface plasmon.

TECHNICAL FIELD

The present invention relates to optical sources, more precisely to thelocalization of light on surfaces or in volumes with one or severaldimensions not limited by the Rayleigh criterion.

BACKGROUND OF THE INVENTION

The localization of light on small dimension surfaces or volumes is ofinterest for many applications. These applications include, but are notlimited to:

-   -   lithography, where the localized fields are used to transfer        specific patterns into a photosensitive medium;    -   optical data storage, where localized sources are utilized to        write or read information in a recording medium;    -   biochips and biosensors, where the interaction of light with        biological or chemical material provides information thereon;    -   microscopy, where the localized field is used to image small        dimension samples.

To increase the resolution and information density in these differentsystems, it is desirable to dispose of light sources localized onarbitrarily small dimensions and not limited by the operationwavelength.

In classical optics, light cannot be focused on dimensions smaller thanthe Rayleigh limit kλ/NA, where λ is the radiation wavelength, NA thenumerical aperture, and k a constant which depends on the type ofimaging system. This is described by M. Born and E. Wolf in “Principlesof Optics”, 6^(th) Ed., Pergamon Press, Oxford, 1980. Within the limitsset by the Rayleigh criterion, there are two possible approaches forproducing a light source with a smaller extension: either reduce thewavelength X or increase the numerical aperture NA. In opticallithography for example, the current trend consists in reducing thewavelength by using light sources in the ultra-violet (UV), deep UV,X-rays or even electron beams, see “The international technology roadmapfor semiconductors”, 2001 Ed., International SEMATECH, 2706 MontopolisDrive, Austin, Tex. 78741, USA.

The Rayleigh limit can also be tackled from both sides, bysimultaneously reducing the wavelength λ and increasing the numericalaperture NA. This is the case in recent developments for optical datastorage, where a short wavelength source in the blue is combined with animmersion lens with a large numerical aperture, see S. Imanishi et al.“Near-field optical head for disc mastering process”, Jpn. J. Appl.Phys., Vol. 39, pp. 800-805 (2000).

However, it is possible to completely overcome the Rayleigh limit byusing near-field optics, see E. Betzig et al. “Breaking the diffractionbarrier: optical microscopy of a nanometric scale”, Science vol. 251,pp. 1468-1470 (1991). In near-field optics, the electromagnetic fieldcan be confined to features much smaller than the wavelength. Actually,the near-field can be localized on dimensions that are even independentof the wavelength and solely determined by the topography of the system,see O. J. F. Martin et al. “Dielectric versus topographic contrast innear-field microscopy”, J. Opt. Soc. Am. A, Vol. 9, pp. 1801-1808(1996). Current techniques for realizing such near-fields include theutilization of small apertures in an opaque screen, see B. Hecht et al.“Scanning near-field optical microscopy with aperture probes:Fundamentals and applications”, J. Chem. Phys., Vol. 112, pp. 7761-7774(2000), or the scattering of light at a sharp tip, see F. Zenhausern etal. “Scanning interferometric apertureless microscopy: optical imagingat 10 Angstrom resolution”, Science, Vol. 269, pp. 1083-1085 (1995).

From a physical point of view, optical near-field effects can be relatedto the depolarization of the electromagnetic field, which takes place atany interface between materials having different electromagneticproperties, e.g. refractive index, permittivity, and/or permeability. Tofulfill the boundary conditions imposed by Maxwell's equations at theinterface, specific components of the incident field can be magnified byan amount proportional to the contrast of the material properties. Forexample, at an interface between two media 1 and 2 with respectivepermittivity ε₁ and ε₂ (assuming ε₁>ε₂ and an incident field propagatingfrom medium 1 into medium 2), the electric field component normal to theinterface in medium 2 has a magnitude larger (by a factor ε₁/ε₂) thanthe electric field incident on the other side of the interface, inmedium 1, see J. D. Jackson “Classical electrodynamics”, 3^(rd). Ed.,Wiley, N.Y., 1999.

In the context of near-field optics, the transmission properties ofmetal holes realized in a plasmon-supporting layer have also recentlyattracted a lot of interest. Several experiments have demonstrated adramatic enhancement of the field transmitted through such holes, see T.W. Ebbeson et al. “Extraordinary optical transmission throughsub-wavelength hole arrays”, Nature, Vol. 391, pp. 667-669 (1998).Applications of this scheme for photolithography and near-field opticallithography have been proposed, see P. A. Wolff U.S. Pat. No. 5,789,742(4 Aug. 1998); T. W. Ebbeson et al. U.S. Pat. No. 5,973,316; T. W.Ebbeson et al. U.S. Pat. No. 6,052,238; and P. R. H. Stark US patentappl. pub. no. 2002/0056816 of 16 May 2002. Note that all theseapplications necessitate one or several holes, with subwavelengthdimension, in the plasmon-supporting layer, which limits the shape andtopology of achievable localized light source. The present inventiondoes not need such holes and thus does not suffer these limitations.

The techniques for structuring the surface of hard or soft materialswith protruding or indented features with one or several dimensions inthe 10-500 nm range are particularly relevant to the present invention.These techniques are well established and form for example the core oftwo different nanofabrication techniques: soft lithography andnano-imprint lithography, see S. R. Quake and A. Scherer “From micro- tonanofabrication with soft materials”, Science, Vol. 290, pp. 1536-1540(2000).

Soft lithography utilizes a mask made of soft material, such as siloxanepolymers, where sub-100 nm features are defined using a mouldingprocess. A structured master, made typically with electron-beamlithography, that has reliefs in a negative image of the desiredpattern, is used as a mould, see Y. N. Xia and G. M. Whitesides “Softlithography” Angew. Chem. Int. Ed. Engl. vol. 33, pp. 550-575 (1998).Soft lithographic masks have been used as phase masks to reproducesub-100 nm features photolithographically, see J. A. Rogers et al.“Using an elastomeric phase mask for sub-100 nm photolithography in theoptical near field”, Appl. Phys. Lett., Vol. 70, pp. 2658-2660 (1997).Similar masks have been used as light coupling structures to pattern aphotoresist with features having an arbitrary shape with dimensions inthe sub-100 nm range, see H. Schmid et al “Light-coupling masks forlensless, sub-wavelength optical lithography”, Appl. Phys. Lett., Vol.72, pp. 2379-2381 (1998). However, it should be noted that these twolithography techniques are limited by the Rayleigh criterion.

For nano-imprint lithography, the mask is no longer soft, but made of ahard material. In that case, the mask also exhibits sub-100 nm featuresand can be fabricated using standard electron beam lithographytechniques, see L. Guo et al “Nanoscale silicon field effect transistorsfabricated using imprint lithography”, Appl. Phys. Lett., Vol. 71, pp.1881-1883 (1997).

A paper authored by M. Paulus and O. J. F. Martin, the inventor,entitled “Light propagation and scattering instratified media: a Green'stensor approach”, J. Opt. Soc. Am. A/Vol. 18, No. 4, April 2001,discusses a technique of computing the electromagentic field thatpropagates and is scattered in three-dimensional structures formed byembedded bodies. Though paper this may help in understanding thephysical background of and even may give rules for determiningappropriate and useful dimensions for implementing the presentinvention, it does not disclose its concept.

In the context of the present invention and its application forlithography, it must be understood that it is not necessary for aphotosensitive layer to be illuminated through its entire thickness.Using for example top surface imaging, it is sufficient that the verytop layer of the system is exposed, see V. Rao et al. “Top surfaceimaging process and materials development for 193 nm and extremeultraviolet lithography”, J. Vac. Sci. Technol. B, Vol. 16, pp.3722-3725 (1998). Therefore, a limited source, even if it does notextend very deep into the photoresist, does not limit the resolutionachievable with a photolithographic process.

Finally, some recent advances in biosensors and biochips relevant to thepresent invention shall be sketched. A biosensor is a device thatconsists of a biological recognition element, or bioreceptor, e.g. anantibody, an enzyme, a protein, a nucleic acid, whole cells, tissues orentire organism. Tremendous progress has been achieved over the last tenyears in the integration of biosensors onto microchips, to createso-called biochips, see T. Vo-Dinh “Nanosensors and biochips: frontiersin biomolecular diagnostics”, Sensors and Actuators B, Vol. 74, pp. 2-11(2001). Working on a nano-metric scale, in addition to increasing theresolution, also provides additional benefits related, for example, toshort diffusion distances, high surface/volume ratios and small heatcapacities.

The interaction of light with biological or chemical material is one ofthe key diagnostic techniques implemented on a biochip. A biochip can becomprised of only the reactive system, or also integrate excitation(illumination, current, etc . . . ) and detection entities. Since abiochip usually consists of arrays of probes used for differentbiochemical assays, a localized light source allows increasing thespatial resolution as well as the overall throughput of the chip.

SUMMARY OF THE INVENTION

The present invention contemplates a different technique and principlethan using apertures for achieving the localization of light on smalldimension surfaces or volumes. It instead uses a surface plasmongenerated at a given materials interface. The electromagnetic fieldassociated with this surface plasmon possesses components which arespecifically enhanced when they encounter an interface between materialswith different electromagnetic properties, e.g. with a differingrefractive index, permittivity, and/or permeability. Geometricalelements such as protrusions or material inconsistencies orinclusionsare used to judiciously position such materials interfaces atthe locations where the light sources must be created. The extension ofthese localized light sources is determined by the extension of the saidprotrusions. In this way, it is possible to localize these light sourceson dimensions that are not limited by the Rayleigh criterion.

The practical effect of this result is the application of the inventionin fields such as optical lithography, optical data storage, biochipsand high resolution optical microscopy.

Further and still other objects of the present invention will becomemore clearly apparent from the following description in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the realization of a localized light source from theinteraction of a surface plasmon with the interfaces of protrudingelements.

FIG. 2 shows the utilization of additional elements to enhance thecoupling of the illuminating field with the surface plasmon.

FIG. 3 describes the possibility of using a stratifiedsurface-plasmon-supporting layer.

FIG. 4 illustrates the utilization of contra-propagating illuminatingfields to enhance the properties of the localized light source.

FIG. 5 shows the utilization of light confinement elements embedded inthe transmission layer.

FIG. 6 illustrates the possibility of addressing individually thedifferent protrusions of a given source using a complexsurface-plasmon-supporting circuit.

FIG. 7 shows the utilization of a localized light source for opticallithography.

FIG. 8 shows a typical field intensity profile obtained from a localizedlight source.

FIG. 9 describes further means of increasing the performance of alocalized light source by using more than one surface-plasmon-supportinglayer.

FIG. 10 illustrates the implementation of a localized light source fordata storage.

FIG. 11 illustrates the combination of a localized light source with adetection mechanism, for data storage.

FIG. 12 shows the utilization of a localized light source in a biochip.

FIG. 13 illustrates the application of a localized light source fornear-field microscopy.

DETAILED DESCRIPTION OF EMBODIMENTS

In the present context, the term “light source” is used to describe asource of electromagnetic radiation in the visible or any other portionof the electromagnetic spectrum.

The novel and innovative scheme makes use of the interaction of light atspecific boundaries or interfaces of the system. At the locations wherethe light source is to be created, there must be a discontinuity of theelectromagnetic properties, e.g. a refractive index, permittivity,and/or permeability change, of the corresponding materials. Thisdiscontinuity leads to discontinuities of specific components of theelectromagnetic field, which produce the said localized light source. Inthe present invention, the plasmon resonance is used to create fieldcomponents which will produce strong discontinuities.

FIG. 1 shows a first embodiment of the present invention. A layer 11which supports surface plasmons is embedded into a stratified mediumcomposed of a substrate 12 and a transmission layer 13. One or severalprotrusions 14 are realized on the transmission layer, forming a kind oftransmitter-localizer structure together with the transmission layer.The protrusions can be made of the same material as the transmissionlayer, or of a different material. The transmission layer and theprotrusions, i.e. the transmitter-localizer structure, may have the sameor different electromagnetic properties, e.g. refractive index,permittivity, and/or permeability. Further, the transmission layer 13may not be present, and the protrusions thus placed directly on or incontact with the plasmon-supporting layer 11. Additional materiallayers, required by the fabrication process or to improve the mechanicalproperties of the structure may be incorporated in the system; a personskilled in the art should have no problem implementing such alterationsor additions.

For visible light operation, the materials which can be used for theplasmon-supporting layer 11 include metals such as gold, silver andcopper. Other metals, such as aluminum, can be used in the UV. Metalsand metal-oxide mixtures such as indium tin oxide, ITO, can be used inthe infrared. Additional materials, including semiconductors, can alsobe used as surface-plasmon-supporting layer.

The substrate 12 can be made of a dielectric material such as glass orpolymer, or of semiconductor material such as silicon or galliumarsenide. The transmission layer 13 or the whole transmitter-localizerstructure can be made of a soft material such as a polymer, or a hardmaterial such as glass or a semiconductor. The protrusions 14 can besimilarly made of soft or hard materials, either the same or differentmaterials as used for the transmission layer 13.

The protrusions 14 have a height in the order of a fraction of theillumination wavelength. Their lateral dimensions vary according to theshape and size of the light source to be produced. Typically, theprotrusions will have at least one lateral dimension smaller than theconsidered illumination wavelength, e.g. in the order of 20-600 nm forvisible and near UV light operation.

At the considered operation wavelength, the electromagnetic properties(e.g. refractive index, permittivity, and/or permeability) of thedifferent materials chosen are such that, when the top surface of thesurface-plasmon-supporting layer is illuminated with the externalilluminating field 15, a surface plasmon 16 is generated at the bottomsurface. This surface plasmon interacts with the protrusions 14 in thestructure. Due to the contrast of the electromagnetic properties (e.g.refractive index, permittivity, and/or permeability) between theprotrusion 14 and the background 17, depolarization fields 18 arecreated. These fields form the localized light sources claimed in thepresent invention. The lateral extension of each light source isdetermined by that of the corresponding protrusion.

Detailed examples for this first and the subsequently describedembodiments follow further below. The usable materials are the same asdescribed above.

FIG. 2 is a second general embodiment of the present invention, whereadditional elements, such as a grating 29, or a waveguide, etc., areused to enhance the coupling of the external illuminating field 25 withthe surface plasmon 26.

FIG. 3 is a third general embodiment of the present invention, where theplasmon-supporting layer 31 is composed of at least two differentmaterials 32 and 33, the composition and arrangement of which are tunedto obtain a surface plasmon 36 for a specific illumination wavelength.

FIG. 4 illustrates the utilization of two contra-propagating externalilluminating fields 45 and 49, which generate on the plasmon-supportinglayer 41 two contra-propagating surface plasmons 46 and 47, therebyproducing a symmetrical light source 48 localized by the protrusions 44.These two illuminating fields may be simultaneously illuminated fromdifferent directions, using two or more coherent or incoherent sources.It may also be illuminated in a non-simultaneous way.

FIG. 5 shows a general embodiment of the invention where the protusions,as e.g. shown in FIG. 1, are replaced by light confining elements 54embedded in the transmission layer 51, where they interact with thesurface plasmon 56. The present invention requires that theelectromagnetic properties e.g. refractive index, permittivity, and/orpermeability, of the confining elements 54 differ from those of thetransmission layer 51. This can be achieved by the inclusion of solid,liquid or gaseous material in the transmission layer 51, by localmodification of the transmission layer 51 using e.g. diffusion throughits surface, and/or by growing the transmission layer between previouslydeposited protrusions 54 in order to obtain a flat surface.

FIG. 6 shows a perspective view of a general embodiment of theinvention, where the plasmon-supporting layer, as e.g. shown in FIG. 1,is replaced by plasmon-supporting strips 61, which can be used toindividually address one or several protruding elements 64 in thesystem, using different illuminating fields. Additional material may beplaced between adjacent plasmon-supporting strips to obtain a planarstructure between the substrate 62 and the transmission layer 63.

The general physical structures described above may be used to advantagein a number of different applications, as described next.

Optical Lithography

FIG. 7 shows an embodiment of the present invention, with a surfaceplasmon supporting layer 71, a substrate 72, a transmission layer 73 andprotrusions 74, which may be of varying dimensions. This structure shallbe called “the mask” in the present context.

The mask is positioned on top of a photosensitive layer 77, which inturn is deposited on an external substrate 79. In the exposition area,contact between the photosensitive layer and the mask occurs only at theprotrusion interfaces. For the localization of light 78 at theseinterfaces between the protruding elements and the photosensitive layer,it is mandatory that their electromagnetic properties, e.g. refractiveindex, permittivity, and/or permeability, differ. It is furtherimportant that the external illuminating light 75 is suited, i.e. has anappropriate wavelength and power, for the creation of the surfaceplasmon 76 and for the patterning of the photosensitive layer 77.

The lateral shape and size of the protrusions 74 define the lateralshape and size of the exposed portions 78 of the photosensitive layer,and thus the subsequently formed structures. The strength of the lightsource in the photosensitive layer is determined by the contrast ofelectromagnetic properties, e.g. refractive index, permittivity, and/orpermeability, between the protrusions 74 and the photosensitive layer77. It is therefore possible to simultaneously expose features ofvarying dimensions. The contrast between exposed and non-exposed regionsdepends on the height of the protrusions 74 on the mask. Note that anegative resist can also be used, together with the embodiment shown inFIG. 4, which produces an inverted localized source.

The following materials and dimensions may be used for the various partsof the device described above.

FIG. 8 shows the electric field intensity in the photosensitive layer,along the line AB in FIG. 7, for three different protrusion widths: w=20nm, w=40 nm, and w=60 nm. These distributions were calculated for theillumination wavelength λ=600 nm. The intensity profiles very wellreproduce the protrusion widths, although their dimensions are muchsmaller than the illumination wavelength.

FIG. 9 shows an embodiment of the invention similar to that in FIG. 7,where an additional, second, plasmon-supporting layer 92 is included inthe system. For specific distances between the first plasmon-supportinglayer 91 and the second one 92, coupled surface plasmon modes 96 can becreated in the system. This coupling mechanism can increase themagnitude and the extension of the localized light sources 98 at theinterfaces between protrusions 94 and photosensitive layer 97.

Regarding materials and dimensions, the above said applies.

Optical Data Storage

FIG. 10 shows an embodiment of the present invention for data storage.The localized light source 108, which may include one or severalprotruding elements 104, is incorporated in a storage head 102, whichmay be fixed or mobile with respect to the recording medium 109. Thestorage head can incorporate additional mechanisms to track therecording medium and control the distance between the recording mediumand the localized light source. This is known in the art and shall notbe described here. The localized light source is used to write and/orread information in the recording medium. In this embodiment, thedetection mechanism 107 is located on the other side of the recordingmedium and the writing/reading processes occur in transmission.

FIG. 11 shows an embodiment of the present invention similar to that inFIG. 10, where the localized light source 118 and the detectionmechanism 117 are located on the same side of the recording medium 119.In this case, the detection mechanism 117 may be combined together withthe localized illumination source 118, in the recording head 112.

Regarding materials and dimensions, the above said applies.

Biochips

FIG. 12 shows an embodiment of the present invention in a biochip. Thesurface-plasmon-supporting layer 121 is now at the bottom and the systemilluminated from underneath with the external illuminating field 125.The protruding areas 124 and the surface plasmon 126 define localizedlight sources 128 on the substrate 129, where the biological or chemicalmaterial 127 is deposited or embedded. The signal resulting from theinteraction of the localized light source with the biological orchemical material may be detected in the far-field or in the near-fieldof the sample.

Regarding materials and dimensions, the above said applies.

High Resolution Optical Microscopy

FIG. 13 shows an embodiment of the present invention as a probe forscanning near-field optical microscopy. The substrate 132 may be formedby the core of an optical fiber or a microfabricated structure, on whichthe surface-plasmon-supporting layer 131 is deposited. When the internalinterface of the surface-plasmon-supporting layer is illuminated withthe external illuminating field 135, a surface plasmon 136 is generatedon the other interface. The interaction of this surface plasmon with oneor several protrusions 134 on the transmission layer 133, generate alocalized light source 138. The protrusions will have at least onedimension smaller than the illumination wavelength. Regarding thematerials for the plasmon-supporting layer, the above said applies.

It is to be understood that the specific embodiments and applications ofthe invention that have been described are merely illustrativeapplications of the principles of the invention. The person skilled inthe art may make numerous modifications to the described methods andapparatus without departing from the true spirit and scope of theinvention.

1. A structure for producing a localized light source in a medium,comprising a source generating incident light, asurface-plasmon-supporting layer, means for transmitting and localizingplasmons between said surface-plasmon-supporting layer and said medium,said transmitter-localizer means including between saidsurface-plasmon-supporting layer and said medium a discontinuity forproviding a localized electromagnetic field deviation and aplasmon-transmitting interface with predetermined electromagneticproperties at said medium wherein said incident light excites a surfaceplasmon in said surface-plasmon-supporting layer, which plasmon in turnproduces said localized light source at said plasmon-transmittinginterface by localizing the energy of said surface plasmon.
 2. Thestructure of claim 1, wherein the discontinuity for providing alocalized electromagnetic field deviation is a physical discontinuitylocalizing the electromagnetic field associated with a plasmon generatedby said surface-plasmon-supporting layer.
 3. The structure of claim 2,wherein the discontinuity consists of or includes one or moreprotrusions contacting the medium.
 4. The structure of claim 2, whereinthe discontinuity consists of or includes of one or more inclusions. 5.The structure of claim 1, further including means, in particular agrating, for enhancing the generation of surface plasmons by thesurface-plasmon-supporting layer.
 6. The structure of claim 1, furtherincluding a substrate carrying the surface-plasmon-supporting layer andthe transmitter localizer, and providing a transfer of the incidentlight.
 7. The structure of claim 1, wherein thesurface-plasmon-supporting layer is made of two or more differentmaterials.
 8. The structure of claim 1, wherein a plurality of sourcesfor generating incident light is provided for simultaneous or sequentialuse.
 9. The structure of claim 1, wherein the surface-plasmon-supportinglayer consists or comprises a plurality of patches or strips which areindividually addressable.
 10. The structure of claim 1, furtherincluding one or more additional surface-plasmon-supporting layers forenhancing the localized light source.
 11. The structure of claim 1,wherein the various layers and elements of said structure arestructured, in particular curved, to enable generating the localizedlight source in one or several locations of the plasmon-transmittinginterface to the medium.
 12. The structure of claim 1, wherein the widthand/or length of the means for localizing the generated plasmon, inparticular of the protrusion, is a fraction of the wavelength of thelocalized light source, preferably less than about one tenth of saidwavelength.
 13. The structure of claim 1, wherein for visible lightoperation, the surface plasmon-supporting layer consists of or includesany of gold, silver and/or copper.
 14. The structure of claim 1, whereinfor operation in the UV region, the surface plasmon-supporting layerconsists of or includes a metal, preferably aluminum.
 15. The structureof claim 1, wherein for operation in the infrared region, the surfaceplasmon-supporting layer consists of or includes a metal and/or ametal-oxyde mixture, preferably indium tin oxide.
 16. A method forgenerating a localized light source in a medium, comprising thefollowing steps: generating incident light, exciting a surface plasmonfrom said incident light in a surface-plasmon-supporting element,transmitting said surface plasmon by plasmon transmission means to alocalized interface with predetermined electromagnetic propertiesbetween said plasmon transmission means and said medium, in which sothat said localized light source is generated at said interface.
 17. Themethod for generating a localized light source according to claim 16,wherein surface plasmons are excited only on the side of the surfaceplasmon-supporting element attached to the plasmon transmission means.18. The method for generating a localized light source according toclaim 16, wherein surface plasmons are excited on both sides of thesurface plasmon-supporting element.
 19. Use of a structure according toclaim 1 in or for optical lithography and/or optical data storage and/orhigh resolution optical microscopy and/or biochips.
 20. Use of a methodaccording to claim 16 in or for optical lithography and/or optical datastorage and/or high resolution optical microscopy and/or biochips.